1. Field Of The Invention
The present invention concerns a laser having a brief discharge between two elongated electrodes.
2. Description Of The Prior Art
In such a laser, the direction of light emission is generally defined mainly by the shape of the space in which the light amplification takes place and not by a resonant optical cavity. The main direction of emission is then that along which the light is amplified for the longest time, i.e. along the channel situated between the two elongated electrodes. The problem is to obtain light emission having as little divergence as possible.
This problem arises more particularly in the case of a laser having a brief discharge of the travelling wave type. Such a laser comprises, conventionally, a flat-plate transmission line with conductive surfaces having a parabolic edge; a spark gap placed at the focus of the parabola short-circuits the two conductive surfaces after the line has been charged and thus generates a cylindrical travelling electric wave front in the line which is converted to a plane wave front by reflection at the edges; a laser channel is formed by interrupting one of the conductive surfaces with a rectilinear slot whose edges constitute the two elongated electrodes and by keeping a gas, such as nitrogen, in that slot. This channel is disposed at an angle to the axis of the parabola; the two electrodes are electrically connected by an induction coil for the charging of the transmission line. A localized electric discharge is thus obtained between the electrodes which moves along the channel at the speed of light and which effects a very rapid excitation of the nitrogen.
This excitation enables the selective amplifying of a travelling light wave accompanying the discharge zone. Such a laser is described in the article by M.B. GODARD - "A simple high-power large efficiency N.sub.2 ultraviolet laser", I.E.E.E. Journal of Quantum Electronics, vol. QE-10 N.degree. 2, February, 1974, pp. 147 to 153.
The electrodes between which the travelling electric discharge appears are polished and have the form of plane parallel surfaces with rounded edges. Such a form is, indeed, simple and conventional for setting up an electric discharge between two electrodes when the firing voltage is required to be well determined and when damaging of the electrodes is to be prevented.
The pulse laser beam thus obtained is not parallel and has a divergence, which can be partly eliminated by placing a lens in the path of the beam having a focus which coincides with a first end of the laser channel, i.e. with the zone in which the electric discharge begins and from which the light wave is subsequently amplified all along the channel. The residual is, however, still a hindrance and cannot be sufficiently corrected by optical means placed in the beam.
Preferred embodiments of the present invention provide a laser having a brief discharge between two elongated electrodes, with a beam of reduced divergence.
The present invention provides a brief discharge laser comprising an enclosure for containing a gas capable of amplifying light when excited by an electric discharge, two parallel elongated electrodes disposed in the enclosure, and having adjacent faces which form two sides of an elongated laser channel running from a first end to a second end, electric means for so generating a brief discharge across the faces of the electrodes that, in operation, a light wave appearing at the first end of the channel and propagating towards the second end is amplified as it passes through excited gas and leaves the enclosure through a suitably placed window in the enclosure, and a convergent optical system placed in the path of light leaving the second end of the channel and having a focus substantially coinciding with the first end of the channel; the shape of said adjacent faces of each of the electrodes as seen in a cross-sectional view taken in a plane perpendicular to the channel being that of a rounded-off point directed towards the other electrode whereby any light propagating obliquely along the channel and being reflected from either electrode is reflected outside the channel and therefore ceases to be amplified during operation of the laser.
A brief discharge travelling wave laser is known, whose electrodes have the form of a rounded point, when seen in a cross-section view taken through a plane perpendicular to the channel containing the active gas. See the British periodical "Opto Electronics" vol. 4 N.degree. 1, February, 1972, pages 43-49, publishing an article by D. Basting et al, "A simple high power nitrogen laser". The laser described therein supplies a beam having great divergence of 66 milliradians in one direction and of 26 milliradians in the other. Its teaching has therefore never been taken into consideration when it was sought to reduce the divergence of the output beam of a laser.
The efficiency of the present invention resides in the fact that the irreducible part of the divergence of known brief discharge lasers was essentially due to the fact that a part of the excitation energy of the gas was used for amplifying light waves which were propagated obliquely in the laser channel by successive reflections on the plane parts of the adjacent faces of the electrodes.